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NASA NTRS1997

Cratering on Titan: A Pre-Cassini Perspective

The NASA-ESA Cassini mission, comprising a formidably instrumented orbiter and parachute-borne probe to be launched this October, promises to reveal a crater population on Titan that has been heretofore hidden by atmospheric haze. This population on the largest remaining unexplored surface in the solar system will be invaluable in comparative planetological studies, since it introduces evidence of the atmospheric effects of cratering on an icy satellite. Here, I highlight some impact features we may hope to find and could devote some modeling effort toward. Titan in a Nutshell: Radius= 2575 km. Density= 1880 kg/cubic m consistent with rock-ice composition. Surface pressure = 1.5 bar. Surface gravity = 1.35 m/square s Atmosphere -94% N2 6% CH, Surface temperature = 94K Tropopause temperature = 70K at 40 km alt. Probable liquid hydrocarbon deposits exist on or near the surface.Titan in a Nutshell: Radius= 2575 km. Density= 1880 kg/cubic m consistent with rock-ice composition. Surface pressure = 1.5 bar. Surface gravity = 1.35 m/square s; Atmosphere about 94% N2 6% CH, Surface temperature = 94K Tropopause temperature = 70K at 40 km alt. Probable liquid hydrocarbon deposits exist on or near the surface. Titan is comparable to Callisto and Ganymede for strength/gravity, Mars/Earth/Venus for atmospheric interaction, and Hyperion, Rhea, and Iapetus for impactor distribution. The leading/trailing asymmetry of crater density from heliocentric impactors is expected to be about 5-6, in the absence of resurfacing. Any Saturnocentric impactor population is likely to alter this. In particular the impact disruption of Hyperion is noted; because of the 3:4 orbital resonance with Titan, fragments from the proto-Hyperion breakup would have rapidly accreted onto Titan. Titan's resurfacing history is of course unknown. The disruption of impactors into fragments that individually create small craters is expected to occur. A crude estimate suggests a maximum separation of about 2 km (compared with 4 km on Venus, or 0.5 km on Earth). Crater chains are unlikely on Titan, since impactors must pass close enough to Saturn to be tidally disrupted; as a result, they would suffer aerodynamic disruption. Crater counting on adjacent satellites gives densities of about 200 per 10 (exp 6) square km for 20-km-diameter craters. However, the presence of a thick atmosphere leads to atmospheric shielding, depleting the relative abundance of small craters. This has been evaluated by models, and the relative abundance of small craters may be due to a diagnostic atmospheric collapse. A number of radar-dark "splotches" have been detected on Venus; these have been attributed to the interaction of the surface with the atmospheric shockwave produced by the Tunguska-like explosion of a bolide in the atmosphere. Simple analogy suggests that similar features might occur on Titan, but the shocked mass density (which controls the momentum coupling between the surface and the shockwave) of Titan's cold N2 atmosphere is about 20x smaller than that of Venus's hot CO2 atmosphere. Unless ice is much more easily turned to rubble than is rock, such features seem less probable on Titan. When the energy deposited by an impact forms a fireball with an equilibrate greater than one scale height, the fireball expands upward and can distribute ejecta. on ballistic exoatmospheric trajectories. On Venus this process is believed to be responsible for the parabolic features; the interaction of various-sized particles falling through the atmosphere with the zonal wind field winnows the particles to form a parabolic deposit. Although such a process is possible on Titan, the large scale height at higher altitudes would make it more difficult. Comparison with craters on other icy satellites suggests that craters on Titan will be fairly shallow (depth/diameter about 0.1) and craters greater than 10 km in diameter will have central peaks or domed bases, perhaps with central pits. The formation of ejecta. blankets may involve the atmosphere in a significant way, both by restraining the expansion of the ejecta cloud and by influencing the thermal history of the ejecta. Compared with Venus, Titan's atmosphere will chill an impact melt somewhat quickly, so the long ejecta flows seen on Venus seem less likely, detailed modeling needs to be performed to determine the impact melt production. Crater topography on Titan may be highlighted by the influence of liquids forming crater lakes. Craters with central peaks will typically form ring-shaped lakes, although horseshoe-shaped takes may be common; domed craters with central pits may even form bullseye lakes with islands with central ponds. If liquids have covered a substantial part of Titan's surface for a substantial period, hydroblemes and tsunami deposits may be common.

Lorenz, R. D.↗
NASA NTRS2000

Orbital Cyclicities Above and Below the Cretaceous-Tertiary Boundary, Umbria-Marche Region, Italy

In the Umbria-Marche region of central Italy, the deep basinal carbonate Scaglia Rossa Formation contains an important sequence of Cretaceous-Tertiary strata including a detailed paleomagnetic record and the distal impactoclastic Cretaceous-Tertiary boundary clay layer. In addition to this significant paleomagnetic and impactoclastic record, the Scaglia Rossa also contains potentially important stratigraphic evidence of relatively long-term oceanic and atmospheric consequences of the Cretaceous-Tertiary bolide catastrophe, which we will describe for the first time herein. Additional information is contained in the original extended abstract.

King, D. T., Jr.↗
NASA NTRS2000

Mineralogical Studies of Experimentally Shocked Dolomite: Implications for the Outgassing of Carbonates

Common rock-forming rhombohedral carbonates - calcite and dolomite - constitute a considerable fraction of terrestrial sediments that may be shocked during hypervelocity impacts, such as during the Cretaceous-Tertiary (K/T) event or at the Haughton structure. The relatively modest temperatures needed to decompose carbonates and to release their CO2 are easily attained during such impacts. However, detailed and quantitative understanding of the CO2 release of carbonates as a function of shock stress is still the subject of controversy, as are a number of other reactions and phase transitions that were suggested for carbonates. The first devolatilization studies of carbonates in the early 1980s suggested that incipient devolatilization of calcite and/or dolomite commences at modest pressures, in the range < 10-18 GPa (amounting to 0.03-0.3% CO2 loss), with massive decarbonation (30-40% CO2 loss) occurring at shock pressure of 20 GPa and above. However, most later studies revealed that both calcite and dolomite are unexpectedly stable under shock conditions and no significant outgassing has been observed at pressures as high as 40 GPa for calcite and 60 GPa for dolomite. Despite these uncertainties, the calculations of O'Keefe and Ahrens (and others) serve to illustrate that the K/T bolide liberated sufficient CO2 that a substantial temperature increase of the global atmosphere is possible due to CO2-triggered greenhouse effects. The quantitative understanding of the devolatilization of carbonates as a function of shock stress is obviously critical to refine such calculations. Additional information is contained in the original extended abstract.

Skala, R.↗
NASA NTRS2001

Impact and Collisional Processes in the Solar System

A series of impact experiments on anhydrite CaSO4, in which vaporized sample accelerates an element in a velocity interferometer, generate velocity data that we have recently reanalyzed using an explicit entropy generating finite difference code. The shock pressure required from the onset, and complete vaporization of 30% porous and 70% crystal density anhydrite is 52 +/- 3 and 122 +/- 13 GPa. Using observed acid leaching in non-marine K/T ejecta in North America, and the sharp global increase in Sr-87/Sr-86 ratios recorded at 65 Ma in marine rocks, we demonstrated that global acidification is primarily due to the SO2 released by anhydrite volatilization, and not HNO3 formed from bolide-induced air pyrolysis. Shock temperatures for crystal CaCO3 are measured from 3000 to 7000 K in the 90 to 160 GPa pressure range. These temperatures are much lower than calculated theoretically indicating that possibly bond breakdown at the shock front is occurring. This is the first mineral in which this effect has ever been seen. New data defining the ion species which are produced upon impact of volatilization of metals and minerals using a pulse ultraviolet laser to simulate intense shock heating from a projectile impact indicate that in shock experiments we can for the first time study the speciation of neutrals using a moderate resolution time-of-flight mass spectrometer. Measurements of the gas species from a series of proposed impact experiments appear to be quite feasible. We will attempt these experiments in the next year. Measurements of the impact induced shock wave decay in SiO2 and GeO2 glass are underway to measure these pure oxide properties. Predictive calculations indicate that the pressure will decay as r(exp -2.7) in the phase transition regime, versus a much lower rate of r(exp -1.18), if a phase transition does not occur.

Ahrens, Thomas J.↗
NASA NTRS2001

Impact Cratering Calculations

We examined the von Mises and Mohr-Coulomb strength models with and without damage effects and developed a model for dilatancy. The models and results are given in O'Keefe et al. We found that by incorporating damage into the models that we could in a single integrated impact calculation, starting with the bolide in the atmosphere produce final crater profiles having the major features found in the field measurements. These features included a central uplift, an inner ring, circular terracing and faulting. This was accomplished with undamaged surface strengths of approximately 0.1 GPa and at depth strengths of approximately 1.0 GPa. We modeled the damage in geologic materials using a phenomenological approach, which coupled the Johnson-Cook damage model with the CTH code geologic strength model. The objective here was not to determine the distribution of fragment sizes, but rather to determine the effect of brecciated and comminuted material on the crater evolution, fault production, ejecta distribution, and final crater morphology.

Ahrens, Thomas J.↗
NASA NTRS2002

The Meteoritic Component in Impact Deposits

An important part of the history of impacts on Earth, and their influence on the terrestrial environment and biotic evolution, is the provenance of the impacting bolides. This will reflect the history of the large-body object flux in the inner solar system. The physical and chemical properties of projectiles, as well as their orbital evolution, has influenced the dynamics and the relative timing of impact events. Possible impact scenarios include random impacts by individual asteroids or comets, or clusters of impacts due to major collisions in the asteroid or Kuiper belts, or large perturbations of the Oort cloud of comets. Over the last several years, a combination of trace element, isotopic, and petrologic data have yielded significant insights into this impact history. The trace element chemistry of sediments, in particular the concentration of siderophiles (e.g., Ir), is a useful tool to detect impacts and provides supporting evidence for suspected impact deposits. However, siderophiles are not especially useful in distinguishing between types of projectiles. Interelement abundances of PGEs can distinguish a chondritic signature, but since most asteroids, and probably all comets are chondritic, these data do little to distinguish between chondritic source materials. Perhaps the most significant chemical argument used to constrain provenance, is that the total amount of Ir in the global Cretaceous-Tertiary (KT) boundary ejecta layer is considerably less than that expected by a low-velocity, 10 km asteroid impact and is most consistent with the impact of a high-velocity, low-Ir comet. Alternatively, much of the Ir may have been buried in the Chicxulub crater and/or ejected to escape velocity.

Kyte, Frank T.↗
NASA NTRS2001

KT Boundary Impact Debris from DSDP Site 577

The KT boundary at DSDP Site 577 (Shatsky Rise, western North Pacific) is in a calcareous nannofossil ooze. It is bioturbated so Ir and impact debris are smeared across less than 15 cm of core. An absence of sulfides and chalcophile enrichment and the presence of Mn-micronodules indicate that this boundary did not experience anoxic conditions typical of many other calcareous KT sites. Because of its shallow burial (109 m) ejecta is extremely well preserved and has been the subject of numerous studies. Three high-temperature minerals have been identified at this site. Clinopyroxene (cpx), found only at Site 577 may be precursor to diagenetic sanidine in some European sites. Magnesioferrite spinel has the highest Fe2O3/FeO of any locality and can have and trace inclusions of Ni-magnesiowustite, which is known at only one other site (Site 596). Other debris described includes shocked quartz grains less than 200 microns, hollow spherules possibly related to impact melt glasses found near Chicxulub, and irregular Ir-rich particles that may be derived directly from meteoritic materials. The latter may be related to the fossil meteorite found at Site 576, just 500 km to the east. A large-volume (approx. 10 cc) sample was taken to systematically characterize debris. The less than 60 microns fraction of the carbonate-free sediment contained 44 mg of impact debris. 2700 particles less than l00 microns were classified based on color, shape, and morphology. About 55% are generally light colored, composed mostly of clays, and typically lack magnesioferrite spinel. Less than half of these are quite spherical; many have irregular shapes, possibly from fragmentation. Hollow spherules with smooth shiny surfaces are only 2% of the total but many fragments in the 60 - 100 microns fraction suggest that they were more abundant and that some may have been quite large; 800 microns or more. Cpx-bearing spherules, easily identified by green crystals at the surface, constitute only 2.5% of the total. Three cpx spherules are dumbbells. The other 40% of the debris is a complex assemblage of dark-colored particles, most of which have magnesioferrite spinel. We have currently separated these into 6 groups based on color and morphology, but this hardly describes the diversity of this group. They range from smooth and spherical to rough and irregular in shape. Some have a planar cleavage, similar to some broken cosmic spherules. 220 particles were irradiated for trace element analysis, as groups and as individual particles. It appears that every spherule type as at least a few ng/g Ir. Several of the spinel-bearing particles have been identified with high Ir (200-1500 ng/g), Cr (2-5 mg/g), and Ni (2-10 mg/g), consistent with results. It is hoped that some of these will yield potential fragments of the KT bolide. Only five have been sectioned to date and they are characterized by high concentrations of spinel; two contained traces of magnesiowustite. This debris constitutes a complex assemblage of ejecta materials. The light-colored, spinel-free particles likely derive mainly from target materials. A significant fraction of these are probably alteration products of cpx-spherules. The dark-colored debris is more likely to have high siderophile concentrations, and thus a high extraterrestrial component. However, the majority don't. Most of the larger spinel-bearing spherules have modest Ir concentrations (5-30 ng/g).

Kyte, Frank T.↗
NASA NTRS2003

Inferred Primary Compositions of Archean Spherules Formed by the Condensation of an Impact-produced Rock Vapor Cloud, Barberton Greenstone Belt, South Africa

Based on the lunar cratering record, impacts were larger and more frequent on the early Earth than they are today. There is no persevered record of these early terrestrial impacts because rocks of this age have been obliterated by tectonism and erosion. The oldest known evidence of impacts on Earth lies in four beds (S1, S2, S3 and S4) in the Barberton Greenstone Belt (BGB), South Africa, ranging in age from about 3.24 to 3.47 Ga. These beds are composed in large part of sand-sized spherical particles, termed spherules, that are thought to have formed by the condensation of rock vapor clouds ejected above the atmosphere as a result of large impacts. Spherule beds S2 and S3 are both about 20 cm thick where composed entirely of fall-deposited spherules and up to a meter thick where spherules are mixed with locally derived debris. The diameters the bolides have been estimated to be between 20 and 50 km, based on bed thickness, size of the largest spherules, Ir fluence and extraterrestrial Cr.

Krull, A. E.↗
NASA NTRS2004

Anhydrite EOS and Phase Diagram in Relation to Shock Decomposition

In the context of the Chicxulub impact, it became recently obvious that experimental and theoretical research on the shock behavior of sulfates is essential for an assessment of the role of shock-released gases in the K/T mass extinction. The Chicxulub crater is the most important large impact structure where the bolide penetrated a sedimentary layer with large amounts of interbedded anhydrite (Haughton has also significant anhydrite in the target). The sulfuric gas production by shock compression/decompression of anhydrite is an important issue, even if the size of Chicxulub crater is only half of the so far assumed size. The comparison of experimental data for anhydrite, shocked with different techniques at various laboratories, reveals large differences in the threshold pressures for melting and decomposition. To gain insight into this issue, we have made a theoretical investigation of the thermodynamic properties of anhydrite. The project includes the review of data published in the last 40 years - reasons to study anhydrite cover a wide field of interests: from industrial problems of cement and ceramic production to the analysis of nuclear underground explosions in salt domes, conducted in the USA and USSR in the 1970th.

Ivanov, B. A.↗
NASA NTRS2005

Offshore Breaking of Impact Tsunami: Van Dorn was Right

Tsunami generated by the impacts of asteroids and comets into the Earth s oceans are widely recognized as a potentially catastrophic hazard to the Earth s population (e.g. Chapman and Morrison 1994, Nature, 367, 33; Hills et al. 1994, in Hazards Due to Comets and Asteroids, (ed. T. Gehrels), 779; Atkinson et al. 2000, Report of the UK Task Force on Potentially Hazardous NEOs; Ward and Asphaug 2000, Icarus, 145, 64). A peculiarity of ocean impacts is the potential global effects of an impact that would otherwise be of only regional or local importance should it occur on land. This is, of course, due to the ability of waves to propagate globally, as seen by the terrible effects of the recent earthquake off the coast of Sumatra. The overall process of an impact tsunami is complex and falls into several distinct phases: 1) initial impact of the bolide into the ocean and formation of a transient cavity in the water, 2) collapse of the cavity and propagation of large waves from the impact center outward over deep water (typically several km in depth), 3) initial effects on wave amplitude as shallower water of the continental slope is reached ("wave shoaling"), possible breaking of waves in relatively shallow water (less than 100 m depth), on continental shelves, and 5) final contact of waves with the shore and their progression onto dry land ("run-up" and "run-in"). Here we report on numerical calculations (and semi-analytic theory) covering phases 3 and 4.

Korycansky, D. G.↗
NASA NTRS2014

Release 2 data products from the Ozone Mapping and Profiler Suite (OMPS) Limb Profiler

The OMPS Limb Profiler (LP) was launched on board the NASA Suomi National Polar-orbiting Partnership (SNPP) satellite in October 2011. OMPS-LP is a limb-scattering hyperspectral sensor that provides ozone profiling capability at 1.5 km vertical resolution from cloud top to 60 km altitude. The use of three parallel slits allows global coverage in approximately four days. We have recently completed a full reprocessing of all LP data products, designated as Release 2, that improves the accuracy and quality of these products. Level 1 gridded radiance (L1G) changes include intra-orbit and seasonal correction of variations in wavelength registration, revised static and intra-orbit tangent height adjustments, and simplified pixel selection from multiple images. Ozone profile retrieval changes include removal of the explicit aerosol correction, exclusion of channels contaminated by stratospheric OH emission, a revised instrument noise characterization, improved synthetic solar spectrum, improved pressure and temperature ancillary data, and a revised ozone climatology. Release 2 data products also include aerosol extinction coefficient profiles derived with the prelaunch retrieval algorithm. Our evaluation of OMPS LP Release 2 data quality is good. Zonal average ozone profile comparisons with Aura MLS data typically show good agreement, within 5-10% over the altitude range 20-50 km between 60 deg S and 60 deg N. The aerosol profiles agree well with concurrent satellite measurements such as CALIPSO and OSIRIS, and clearly detect exceptional events such as volcanic eruptions and the Chelyabinsk bolide in February 2013.

Limb scatter↗
NASA NTRS2015

When the Sky Falls: Performing Initial Assessments of Bright Atmospheric Events

The 2013 Chelyabinsk super bolide was the first "significant" impact event to occur in the age of social media and 24 hour news. Scientists, used to taking many days or weeks to analyze fireball events, were hard pressed to meet the immediate demands (within hours) for answers from the media, general public, and government officials. Fulfilling these requests forced many researchers to exploit information available from various Internet sources - videos were downloaded from sites like Youtube, geolocated via Google Street View, and quickly analyzed with improvised software; Twitter and Facebook were scoured for eyewitness accounts of the fireball and reports of meteorites. These data, combined with infrasound analyses, enabled a fairly accurate description of the Chelyabinsk event to be formed within a few hours; in particular, any relationship to 2012 DA14 (which passed near Earth later that same day) was eliminated. Results of these analyses were quickly disseminated to members of the NEO community for press conferences and media interviews. Despite a few minor glitches, the rapid initial assessment of Chelyabinsk was a triumph, permitting the timely conveyance of accurate information to the public and the incorporation of social media into fireball analyses. Beginning in 2008, the NASA Meteoroid Environments Office, working in cooperation with Western's Meteor Physics Group, developed processes and software that permit quick characterization - mass, trajectory, and orbital properties - of fireball events. These tools include automated monitoring of Twitter to establish the time of events (the first tweet is usually no more than a few seconds after the fireball), mining of Youtube and all sky camera web archives to locate videos suitable for analyses, use of Google Earth and Street View to geolocate the video locations, and software to determine the fireball trajectory and object orbital parameters, including generation of animations suitable for popular media and presentations. This presentation will give a short description of the characterization procedure and show applications of the tools, which have become vital to answering the question of "What was that bright light in the sky?" in the post-Chelyabinsk, 24/7 news world.

Cooke, William J.↗
NASA NTRS2016

A Fragment-Cloud Model for Breakup of Asteroids with Varied Internal Structures

As an asteroid descends toward Earth, it deposits energy in the atmosphere through aerodynamic drag and ablation. Asteroid impact risk assessments rely on energy deposition estimates to predict blast overpressures and ground damage that may result from an airburst, such as the one that occurred over Chelyabinsk, Russia in 2013. The rates and altitudes at which energy is deposited along the entry trajectory depend upon how the bolide fragments, which in turn depends upon its internal structure and composition. In this work, we have developed an analytic asteroid fragmentation model to assess the atmospheric energy deposition of asteroids with a range of structures and compositions. The modeling approach combines successive fragmentation of larger independent pieces with aggregate debris clouds released with each fragmentation event. The model can vary the number and masses of fragments produced, the amount of mass released as debris clouds, the size-strength scaling used to increase the robustness of smaller fragments, and other parameters. The initial asteroid body can be seeded with a distribution of independent fragment sizes amid a remaining debris mass to represent loose rubble pile conglomerations, can be given an outer regolith later, or can be defined as a coherent or fractured monolith. This approach enables the model to represent a range of breakup behaviors and reproduce detailed energy deposition features such as multiple flares due to successive burst events, high-altitude regolith blow-off, or initial disruption of rubble piles followed by more energetic breakup of the constituent boulders. These capabilities provide a means to investigate sensitivities of ground damage to potential variations in asteroid structure.

Asteroid↗
NASA NTRS2019

The Seismic Signatures of Recently Formed Impact Craters on Mars

We investigated the seismic signatures of recent impact crater clusters on Mars that would be recorded by the Interior Exploration using Seismic Investigations, Geodesy and Heat Transport (InSight)seismometers. We used a database of 77 measured and dated impact sites, with craters with diameters between 2.1 and 33.8 m, along with inferred impact angle, bolide trajectory, and varying target material properties to empirically scale for the momentum, expected seismic source function, and radiation pattern of impacts. The impact source is simulated in a local 3‐Dfinite difference wave propagation code and coupled to teleseismic distances by scaling the spectra of 1‐D global synthetic seismograms. We use the InSight seismometer noise floors to estimate detectability of impact(s) across azimuth and distance. Our experiments reveal that impact clusters have a higher peak corner frequency resulting from energy contributed by smaller craters to the power spectrum. We also find that the time separation between individual impacts in a cluster is small (<10–15 ms) and requires a seismometer closely situated to the source (<10 km) and a high sampling rate (>100 Hz) to resolve individual impacts within the cluster. Two of the clusters in our database(>20‐m effective diameter) would have been detectable by InSight, with the assumptions that the Martian background noise and seismic attenuation are both low. Joint detection of surface changes from newly formed crater(s) in images and by InSight will provide precise source locations that are crucial for constraining the internal structure of Mars

N. C. Schmerra↗
NASA NTRS2018

Impact-Seismic Investigations of the InSight Mission

Impact investigations will be an important aspect of the InSight mission. One of the scientific goals of the mission is a measurement of the current impact rate at Mars. Impacts will additionally inform the major goal of investigating the interior structure of Mars. In this paper, we review the current state of knowledge about seismic signals from impacts on the Earth, Moon, and laboratory experiments. We describe the generalized physical models that can be used to explain these signals. A discussion of the appropriate source time function for impacts is presented, along with spectral characteristics including the cutoff frequency and its dependence on impact momentum. Estimates of the seismic efficiency (ratio between seismic and impact energies) vary widely. Our preferred value for the seismic efficiency at Mars is 5 × 10−4, which we recommend using until we can measure it during the InSight mission, when seismic moments are not used directly. Effects of the material properties at the impact point and at the seismometer location are considered. We also discuss the processes by which airbursts and acoustic waves emanate from bolides, and the feasibility of detecting such signals. We then consider the case of impacts on Mars. A review is given of the current knowledge of present-day cratering on Mars: the current impact rate, characteristics of those impactors such as velocity and directions, and the morphologies of the craters those impactors create. Several methods of scaling crater size to impact energy are presented. The Martian atmosphere, although thin, will cause fragmentation of impactors, with implications for the resulting seismic signals. We also benchmark several different seismic modeling codes to be used in analysis of impact detections, and those codes are used to explore the seismic amplitude of impactinduced signals as a function of distance from the impact site. We predict a measurement of the current impact flux will be possible within the timeframe of the prime mission (one Mars year) with the detection of ∼ a few to several tens of impacts. However, the error bars on these predictions are large. Specific to the InSight mission, we list discriminators of seismic signals from impacts that will be used to distinguish them from marsquakes. We describe the role of the InSight Impacts Science Theme Group during mission operations, including a plan for possible night-time meteor imaging. The impacts detected by these methods during the InSight mission will be used to improve interior structure models, measure the seismic efficiency, and calculate the size frequency distribution of current impacts.

Ingrid Daubar↗
NASA NTRSDate not supplied

Large Scale Volcanism: An Explanation for the Heat-death of Venus-like Worlds

Large scale volcanism has played a critical role in the long-term habitability of Earth and possibly Venus. Large Igneous Provinces (LIPs) are in fact the cause of most major extinction events in Earth’s history, contrary to the popular belief about the devastation from large bolides. We examine the timing of Large Igneous Provinces (LIPs) through Earth’s history [1] to estimate the likelihood of nearly simultaneous events that could drive a planet into an extreme moist or runaway greenhouse, quenching subductive plate tectonics. Such events would end volatile cycling and may have driven Venus from a cool temperate state into the heat-death we see today. Using the Earth's LIP record [1] we have found that LIP events are distributed randomly throughout the past 3 billion years. A conservative estimate of the rate of LIPs in a random history statistically the same as Earth’s, pairs and triplets of LIPs closer in time than 0.1-1 million years are likely. This simultaneity threshold is significant to the extent that it is less than the time over which environmental effects have been shown to persist, for example in the Siberian Traps record [2,3]. With the recent mission selections of VERITAS and EnVision it may be possible for their high resolution radar maps to distinguish LIPs in the surface record we can see today on Venus.

Large Igneous Provinces↗
NASA NTRS2018

The Paleomineralogy of the Hadean Eon Revisited

A preliminary list of plausible near-surface minerals present during Earth’s Hadean Eon (>4.0 Ga) should be expanded to include: (1) phases that might have formed by precipitation of organic crystals prior to the rise of predation by cellular life; (2) minerals associated with large bolide impacts, especially through the generation of hydrothermal systems in circumferential fracture zones; and (3) local formation of minerals with relatively oxidized transition metals through abiological redox processes, such as photo-oxidation. Additional mineral diversity arises from the occurrence of some mineral species that form more than one ‘natural kind’, each with distinct chemical and morphological characteristics that arise by different paragenetic processes. Rare minerals, for example those containing essential B, Mo, or P, are not necessary for the origins of life. Rather, many common minerals incorporate those and other elements as trace and minor constituents. A rich variety of chemically reactive sites were thus available at the exposed surfaces of common Hadean rock-forming minerals.

Shaunna M. Morrison↗
NASA NTRS2017

Physical Properties of the Stone Meteorites: Implications for the Properties of Their Parent Bodies

The physical properties of the stone meteorites provide important clues to understanding the formation and physical evolution of material in the Solar protoplanetary disk as well providing indications of the properties of their asteroidal parent bodies. Knowledge of these properties is essential for modeling a number of Solar System processes, such as bolides in planetary atmospheres, the thermal inertia of atmosphereless solid body surfaces, and the internal physical and thermal evolution of asteroids and rock-rich icy bodies. In addition, insight into the physical properties of the asteroids is important for the design of robotic and crewed reconnaissance, lander, and sample return spacecraft missions to the asteroids. One key property is meteorite porosity, which ranges from 0% to more than 40%, similar to the range of porosities seen in asteroids. Porosity affects many of the other physical properties including thermal conductivity, speed of sound, deformation under stress, strength, and response to impact. As a result of the porosity, the properties of most stone meteorites differ significantly from those of compact terrestrial rocks, whose physical properties have been used in many models of asteroid behavior. A few physical properties, such as grain density, magnetic susceptibility, and heat capacity are not functions of porosity. Taken together, the grain density and the magnetic susceptibility can be used to classify unweathered or minimally weathered ordinary chondrites. This provides a rapid screening technique to identify heterogeneous samples, classify new samples, and identify misclassified meteorites or interlopers in strewn fields.

Chondritic meteorites↗